1. Technical Field
The present disclosure relates generally to torque limiters and more specifically to a torque limiter suitable for use in surgical procedures.
2. Background Information
It is increasingly common for a surgeon or other medical practitioner to insert fasteners into the human body to promote proper healing. For example, orthopedists often treat a variety of different types of bone fractures and other skeletal conditions by installing bone screws, rod and cap screws, or other fasteners in the affected area, to stabilize the bone, to secure implants, or for other therapeutic purposes. Bone screws or other fasteners may require predrilled and tapped holes, or may be self-drilling and self-tapping. When installing a screw or other fastener into bone, it is important that the bone not be subject to excessive torque. Excessive torque may “strip” threads in the bone, or may otherwise damage the bone. Further excessive torque may damage screws or other fasteners, for example by stripping the head of a fastener made from a delicate bioabsorbable material. Thus, applying a limited amount of torque is important in the surgical setting.
A variety of different type of torque limiters exist that are commonly used in other fields to limit torque applied by a driving device, for example a manual tool or a power tool, to a load. Existing torque limiters may be broadly classified into two types: disconnect types that uncouple the driving device from the load, such that little or no residual torque makes its way to the load once a particular torque limit is reached, and torque reduction types which slip (i.e. let a drive component rotate at a different rate from the load) such that, while some torque is still applied to the load, it does not exceed the torque limit.
Disconnect torque limiters may be further classified into a variety of subtypes. One subtype, shear torque limiters, operate by sacrificing a shear-pin, shear-ring or other mechanical component when the torque limit is reached, breakage of the component disconnecting a driving device from the load. Another subtype, ball detent torque limiters, operate by transmitting force through hardened balls which rest in detents. The balls are often held in place by springs. When the torque limit is reached, the balls are pushed out of their detents, disconnecting the driving device from the load. The balls are often retained in a secondary non-load bearing position until they are manually reset into the detents. Yet another subtype, pawl and spring torque limiters, operate by using a spring to hold a drive pawl against a notch in a rotor. When the torque limit is reached, the drive pawl disengages from the notch, thereby disconnecting a driving device from the load.
Torque reduction torque limiters may be also classified into a variety of subtypes. The most common subtype, friction torque limiters, include two friction plates or other friction elements that are compressed and grip against one another. The limit of the torque applied is a result of the coefficient of friction between the plates. If excessive torque is applied, the plates simply begin to slip.
While existing torque limiters may be acceptable for certain applications, they are poorly suited for use in surgical procedures, such as installing screws or other fasteners into bone. Many existing torque limiters are not resilient to certain sterilization techniques. For example some existing torque limiters require, or operate best with, lubricating oils that degrade at high temperatures. Thus, such torque limiters are often incompatible with autoclaves, which typically use high-temperature steam to sterilize surgical instruments. Other designs may not be compatible with ethylene oxide (ETO) and/or Gamma sterilization. Many existing torque limiters are also quite complicated. Such complexity may lead to unacceptable size and weight characteristics, increased occurrence of failures and malfunctions, and high costs. Indeed, many existing torque limiters are quite unsuited for single-use applications given their complexity and resultant cost. Finally, many existing torque limiters are not as accurate and precise as desired at the low torque levels commonly used in surgical procedures.
Accordingly, there is a need for an improved torque limiter suitable for use in surgical procedures that overcomes the shortcomings of prior designs.